1. Field of the Invention
The invention relates to a system implementing a method for retrieving data bits of a data signal in the form of digital sampling values on which the data bits are modulated asynchronously at a known modulation frequency, the system receiving a phase signal supplying the phase position of the data bits of the sampled data signal relative to a predetermined starting point.
2. Description of the Related Art
A known data signal that includes data bits that are asynchronously modulated at a given modulation frequency may be, for example, a teletext signal. Such a teletext signal occurs in given picture lines of a video signal. It is true that its modulation frequency is known, but the phase position of the data bits of the teletext signal within the television lines is not known. This phase position may, however, be determined and generated in a corresponding phase signal that indicates the phase position of the data bits. Furthermore, it is assumed that the data signal, i.e., for example, the teletext signal, is present in a sampled form, i.e., in the form of individual digital sampling values.
A known system for decoding data in the vertical blanking interval, which conventionally occurs in television signals, is known from U.S. Pat. No. 5,555,025, in which the type of data in the signal the frequency and the phase position of this signal are determined in a relatively elaborate way. A clock is then generated, which is formed in such a way that the sampling of the analog data signal takes place in a synchronized form with respect to the phase position of the bits. In this case, the sampling frequency is thus oriented in accordance with the modulation frequency or the phase position of The data bits. This procedure is very elaborate and not very useful in many receiver concepts, because the sampling frequency must be varied and, as is normally useful and relatively simple, cannot be coupled either to the chrominance subcarrier frequency or to the line frequency of the video signal.
It is an object of the invention to provide a system that, on the one hand, does not require a coupling of the sampling frequency with the phase position and/or the modulation frequency of the data bits of the data signal and, on the other hand, requires a small number of component Moreover, at least elements of system should be realizable in software.
According to the invention, this object is solved in that the system determines the positions of the data bits to be retrieved from the phase signal and the known modulation frequency of the data bits, in that the system determines for each data bit to be retrieved whether its position is located around the position of a sampling value within a tolerance window of predetermined width, in that the system, if this should be the case, uses the value of the sampling value within the tolerance window for retrieving the data bit, and in that the system, in the case where the position of a data bit to be retrieved is not located around the position of a sampling value within a tolerance window, uses both sampling values on both sides of the position of the data bit to be retrieved for retrieving the data bit.
Since, on the one hand, the modulation frequency of the data bits is known and, on the other hand, the position of the data bits is known from the phase signal applied to the system, the position of every individual data bit can be determined therefrom. Thus, the system determines, from this information, those temporal positions at which data bits occur. The central position of a data bit is then each time concerned.
Since it is fundamentally assumed in the invention that the sampling frequency is not coupled with the modulation frequency of the data bits, it cannot be assumed that these positions of the data bits exactly correspond to the positions of the sampling values. Actually, there will even be a mutual drift of these positions so that a central position of a data bit is rather accidentally hit by the position of a sampling value.
Thus, there is problem of how to use suitable sampling values for the determined positions of the data bits. The system according to the invention solves this problem in that tolerance windows are used which are positioned around the positions of the sampling values. These tolerance windows may be chosen with a predetermined width, but they may also be variable.
For each data bit to be retrieved, and whose position was determined, it is now determined whether this position is located around the position of a sampling value within a tolerance window. When this is the case, it can be assumed that the value of the sampling value, whose tolerance window includes the data bit to be retrieved, closely approximates the actually desired value of The data bit. This is also possible because, based on the sinusoidal variation of such signals, the difference between these two values will be very small as they are in a range of a maximum or a minimum in which the variation of a sine curve is relatively flat.
When the position of a data bit to be retrieved is located around the position of a sampling value within the tolerance window, the value of this sampling value is used for determining the bits to be retrieved.
In the reverse case, in which the position of a data bit to be retrieved is not located around the position of a sampling value within a tolerance window, this possibility does not usefully and immediately exist. Instead, the two sampling values located on both sides of the position of the data bit to be retrieved are used for retrieving the data bit. These two sampling values will be relatively close to the value of the searched data bit to be retrieved and may therefore be usefully employed for determining the data bit to be retrieved. Based on the computed phase position, further samples between the original sampling values are referred to. The samples may be interpolated in a simple manner.
The essential advantage of this system according to the invention is that its mode of operation is very simple. Independent of the fact whether it is realized in hardware or software, the determination of the data bits to be retrieved or the determination of their value requires only a very small number of components, because sampling values which are present can either be directly taken over, or such a data bit to be retrieved is determined from two sampling values. The computation effort and the memory access are thus minimal. Moreover, the system does not impose any requirements on the selection of the sampling frequency, except of course compliance with the Nyquist criterion. It may be particularly uncoupled with all other frequencies, for example, with the modulation frequency. The system is thus very insensitive to disturbances and suitable for universal use.
One embodiment of the invention ensures that the two sampling values, which, as described above, are located on both sides of the position of the data bits to be retrieved, are used for retrieving the data bit in such a way that a new supporting value is formed in accordance with the phase position from the two sampling values, which supporting value is used as a value for the data bit to be retrieved and from which the data bit can be determined, all this in the case where the position of a data bit to be retrieved is not located around the position of a sampling value within a tolerance window. This requires a minimal computation effort because only additions and divisions by 2n are required, in which the division by 2n can be realized by means of a single shift in the binary system.
To further improve the detection security, a further embodiment of the invention provides adaptive clipping levels. These clipping levels are used to find the value of the data bits so as to recognize the data bits to be retrieved. When a given clipping level is exceeded, a logic first value is concerned and when the value falls below another clipping level, a logic second value is concerned. The adaptive implementation of the clipping level may be preferably oriented on the maximum amplitude of the sampling values. In this way, for example, possible DC drifts may be taken into account in the choice of the values for the clipping level.
In a further embodiment of the invention, the width of the tolerance bands can be advantageously formed adaptively, which width can be adjusted in dependence upon the desired detection security. When the data signal is of a type comprising an error correction, it can be determined with reference to the occurring errors in the evaluation of the detected data bits of the data signal how many errors are involved. Dependent on this error rate, the width of the tolerance band can be adjusted in such a way that, maximally, a desired error rate occurs.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.